In HPLC, substances are separated by being rinsed under high pressure through a column with the aid of a solvent. Pumps which can deliver a constant flow rate continuously at high pressure are required for this. In order to achieve more rapid or better dissolved separations in HPLC, the trend is toward even higher pressures of 1500 bar and more.
An HPLC system contains a plurality of components which have to be fluidically connected to one another and sealed off from one another. For the connection, use is made of capillary tubes which are connected to standardized bushing units. For some components, however, direct assembly in a small construction space is desired. This is true, for example, of nonreturn valves or filters. A different method for ensuring a tight connection has to be used therefor.
In HPLC, piston pumps are used for producing the liquid pressure. These pumps require at least one inlet valve and one outlet valve which have to be connected to the pump head in which the working cylinder is located. There are particularly stringent requirements regarding the load-bearing capacity for the connections between the pump head and the valves, since said connections are exposed not only to the relatively constant system pressure at the pump outlet but also, during each piston stroke of the pump, to a continuous change between ambient pressure and system pressure.
In order to achieve a good compression ratio, the liquid volume in the connection and in the supply lines has to be as small as possible. This is because a dead volume in the supply lines or connections increases the overall volume, and therefore, for the same piston stroke, the pump is able to produce only a smaller system pressure or—in the case of a predetermined system pressure, only a smaller delivery volume per piston stroke; this is because some of the piston stroke is required for compression of the liquid until the system pressure is reached.
A suitable connection for such components of HPLC should not have too high a flow resistance on the inlet side so that rapid intake of the required liquid volume is possible. A minimum inside diameter must therefore be observed.
In addition, when valves and pumps are connected, the connection frequently has to have a short overall length in order to be able to form the entire pump unit as compactly as possible.
Finally, the press-on force required for producing the connection should be able to be applied in a simple manner, preferably without a tool. Screw connections, the interacting threads of which have a relatively large diameter and a relatively small pitch because of the small axial extent as a consequence of the shortest possible overall length, are suitable therefor. Screw connections of this type can also be closed manually even under the high sealing pressure which can be applied.
Known seals of valves have generally annular sealing elements made of PEEK or of another elastomer, said sealing element sealing off the components of the valve on the end side in relation to the valve housing and also sealing off the valve on the end side in relation to the components lying therearound. Said seals are still reliable at 1000 bar. However, at higher pressures, in particular the sealing element on the inlet side of the valve begins to extrude during operation, and thus loses prestress and becomes untight.
Valves according to the prior art which have a housing consisting entirely of metal are sealed off either with PEEK flat seals or with a conical or curved metal seal. In the case of the flat seals, very exacting requirements are imposed on the surface quality of the metal surfaces and very high prestress is required, said prestress also having to be present even under maximum liquid pressure. This is not entirely simple because the small thickness of a flat seal provided on the end side means that said seal has scarcely any elasticity (or only a small elastic spring deflection). Metal seals are extremely sensitive to small impurities, and there is the risk that, in the case of repeated installation, not only the valve, but also the sealing surface of the pump head will be damaged.
FIG. 5 in DE 10 2008 059 897 A1 illustrates one possibility of how a filter can be connected directly to a bushing unit designed for capillary tubes. However, disadvantages in this case include the fact that said connection requires a relatively large number of components and requires a relatively large overall length. An additional plug housing for prestressing the seal is required. The force cannot be applied directly by means of the filter because then the tube rather than the seal would press onto the bushing unit. In addition, it is virtually impossible to manufacture the filter with the required long capillary tube in one piece because an extremely long and thin hole would have to be drilled. The tube would have to be even longer than that illustrated in FIG. 5 of DE 10 2008 059 897 A1 since, for the production of the application region, a tool has to be fitted from behind. It is also proposed here to weld an extruded capillary tube to the structural element, which is associated with a corresponding outlay in terms of production technology.
For high-pressure connections outside HPLC, metal tubes with conical ends are generally pressed into matching conical counterparts. The metal-seal disadvantages already referred to are present in this case.
Within the pressure range of up to approx. 20 bar, O rings which partially fill a groove are used for compressed-air and water-hose connections. Said O rings are prestressed by the mounting thereof and, upon application of a pressure in the medium, are pressed further in the direction of the gap which is to be sealed. However, within the high-pressure range at up to 2000 bar, the deformation of the sealing ring that takes place because of elasticity and compressibility is a serious problem. In the event of a dynamic load, the continuous movement of the seal can result in mechanical wear due to abrasion. In addition, there is a substantial deterioration in the compression ratio of the pump if the sealing ring also has to be compressed in addition to the medium.